This invention relates to a time based mass flow controller (MFC) and a method for controlling volume/mass flow rate accurately based on time.
MFCs are used in various fields such as electronic equipments, fluid equipments, etc. For instance, the manufacturing processes for semiconductor devices utilize the deposition device, the diffusion device, the etching device, the plasma device, and the sputtering device. Since each process uses different kinds of gases, all of them need to accurately and independently control the amount of gas or steam supplied. Furthermore, the remarkable size reduction of semiconductor devices through the improvement of integration techniques requires the supply gas flow rate to be controlled more accurately.
Flow rates are very difficult to measure directly. As disclosed in U.S. Pat. Nos. 6,044,701, 5,279,154 and 4,686,856 and Japanese Unexamined Patent Publication No. s59-88622, various kinds of flow controllers and meters have been suggested and used. Nevertheless, most of them are complex and expensive in installation and maintenance. Namely, the conventional MFCs are highly specialized and expensive equipments.
The conventional MFCs can be classified mainly into the thermal MFCs, which work by detecting the difference in the amount of heat transfer according to a calorimetric principle, and the pressure-based ones, which work on the relation between mass flow and pressure loss in a flow passage.
The thermal MFCs tend to have a relatively slow response time and are not accurate enough because of heat loss to the outside. Also, they require very accurate mechanical parts such as a bypass pipe and cannot be used with sensitive gases. Therefore, it isn""t east to manufacture them. The pressure based MFCs require very accurately engineered components, and can suffer from xe2x80x98foulingxe2x80x99 of the flow passage such as an orifice or a nozzle.
This invention is achieved to solve the above problem. It is an object of this invention to provide a novel MFC that accurately and easily measures flow, based on a different concept from the conventional MFCs. For instance, since time can be measured more accurately than any other physical value, by using a quartz crystal timing device, this invention can easily provide accurate MFCs without an accurate implemental structure. It is another object of this invention to provide a MFC with excellent reliability, longevity, robustness, and inexpensive installation and maintenance. It is now inexpensive and easy to measure time very accurately (better than {fraction (1/10)}8 sec). Therefore, this invention is characterized in that the mass/volume flow rate can be controlled very accurately based on time. To achieve the above objects, this invention provides a time based mass flow controller comprising: at least one reservoir; at least one inlet valve, each inlet valve opening/closing an inlet of each corresponding reservoir; at least one outlet valve, each outlet valve opening/closing an outlet of each corresponding reservoir; at least one pressure sensor, each pressure sensor detecting the gas pressure in each corresponding reservoir; and a controller for controlling the number of opening/closing loops of the inlet valve and the outlet valve per unit time and/or mass/volume of a gas stored and discharged at the reservoir per unit opening/closing loop of the inlet valve and the outlet valve, with the inlet valve opening the inlet of the reservoir to allow the gas to flow into the reservoir when the outlet valve is in a closed state, and thereafter the outlet valve opening the outlet of the reservoir to allow the gas to flow out from the reservoir when the inlet valve is in a closed state, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
The inlet valve and the outlet valve are required to have fast acting performance at the time of their opening/closing movement, like a fast acting valve. The control of the valves can be obtained by utilizing an exclusive logic digital machine, which can control the operation of the fast acting valves based on the pressure in the reservoir.
According to a preferred embodiment of this invention, the controller controls the inlet valve to open the inlet of the reservoir when the reservoir is in a state of a reference empty pressure and to close the inlet of the reservoir in a state of a reference fill pressure, and the controller controls the outlet valve to close the outlet of the reservoir when the reservoir is in a state of the reference empty pressure and to open the outlet of the reservoir when the inlet valve is in a closed state. The controller can control a time period from an opening of the inlet of the reservoir to a closing of the inlet of the reservoir and/or from a closing of the outlet of the reservoir to a opening of the outlet of the reservoir, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
According to another preferred embodiment of this invention, the reservoir is a volume-variable storage chamber and the controller controls the volume of the reservoir.
For example, the reservoir takes a piston-cylinder form, and the controller controls the inlet valve to open the inlet of the reservoir when the reservoir is in a state of a reference empty pressure and to close the inlet of the reservoir when the reservoir is in a state of a reference fill pressure, and the controller controls the outlet valve to close the outlet of the reservoir when the reservoir is in a state of the reference empty pressure and to open the outlet of the reservoir when the reservoir is in a state of the reference fill pressure, and the controller controls the displacement of the piston of the reservoir to control the volume of the reservoir, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
According to still another preferred embodiment of this invention, the controller controls the properties, such as the temperature of the gas flowing into the inlet of the reservoir. These allow the fluid passing through the MFC to have a required characteristic.
Preferably, the time based mass flow controller further comprises a regulator on an upstream side of the inlet of the reservoir, the regulator regulating the gas pressure of an inflow gas into the reservoir to be constant. And, the time based mass flow controller further comprises a main valve on an upstream side of the inlet of the reservoir for regulating the mass/volume flow rate of an inflow gas into the reservoir, wherein the controller controls the inlet valve to open the inlet of the reservoir when the reservoir is in a state of the reference empty pressure and to close the inlet of the reservoir when the reservoir is in a state of the reference fill pressure, and the controller controls the outlet valve to close the outlet of the reservoir when the reservoir is in a state of the reference empty pressure and to open the outlet of the reservoir when the reservoir is in a state of the reference fill pressure, and the controller controls the opening level of the main valve to control the mass/volume flow rate of the inflow gas into the reservoir, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
According to still another preferred embodiment of this invention, a time delay is provided between a closing of the inlet valve and an opening of the outlet valve and/or between a closing of the outlet valve and an opening of the inlet valve and the controller controls the length of the time delay, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
According to still another preferred embodiment of this invention, the time based mass flow controller further comprises a temperature sensor for measuring the gas temperature in the reservoir, wherein the controller controls the number of opening/closing loops of the inlet valve and the outlet valve per unit time and/or mass/volume of gas stored and discharged at the reservoir per unit opening/closing loop of the inlet valve and the outlet valve, based on the gas temperature in the reservoir measured by the temperature sensor, so that the controller may control the mass/volume flow rate of the outflow gas from the reservoir.
According to still another preferred embodiment of this invention, more than one reservoir is installed in parallel and each of the inlet valves and the outlet valves of the respective reservoirs have different opening/closing points in time.
According to still another preferred embodiment of this invention, more than one reservoir is installed in parallel, and the length of the flowing line from each outlet valve to the gas user equipment could be different. And, more than one regulator can be installed in parallel, each regulator being installed on an upstream side of the inlet of each corresponding reservoir and regulating the gas pressure of an inflow gas into each corresponding reservoir to be constant. Also, more than one buffer can be installed in parallel, each buffer being installed on a downstream side of the outlet of each corresponding reservoir and having a large volume than each corresponding reservoir.
According to still another preferred embodiment of this invention, the time based mass flow controller further comprises a buffer installed on a downstream side of the outlet of the reservoir, the buffer having a larger volume than the reservoir. Preferably, the buffer has a spring means, the spring means acting against the gas pressure in the buffer. For example, the spring means has a movable wall and a spring, the movable wall having the interior face on which the gas pressure in the buffer is applied and the exterior face on which the spring acts against the gas pressure in the buffer. For another example, the spring means takes a piston form and the controller controls a displacement of the piston according to the pressure of the gas in the buffer.
Also, this invention provides a method for controlling the mass/volume flow using a time based mass flow controller, the time based mass flow controller comprising at least one reservoir, at least one inlet valve, each inlet valve opening/closing an inlet of each corresponding reservoir, at least one outlet valve, each outlet valve opening/closing an outlet of each corresponding reservoir, and at least one pressure sensor, each pressure sensor detecting the gas pressure in each corresponding reservoir, the method comprising: controlling the number of openings/closings of the inlet valve and the outlet valve per unit time and/or mass/volume of a gas stored and discharged at the reservoir per unit opening/closing loop of the inlet valve and the outlet valve, with the inlet valve opening the inlet of the reservoir to allow the gas to flow into the reservoir when the outlet valve is in a closed state, and thereafter the outlet valve opening the outlet of the reservoir to allow the gas to flow from the reservoir when the inlet valve is in a closed state, so as to control the mass/volume flow rate of the outflow gas from the reservoir.